Radio frequency (RF) equipment uses a variety of approaches and structures for receiving and transmitting radio waves in selected frequency bands. Typically, filtering structures are used to maintain proper communication in frequency bands assigned to a particular band. The type of filtering structure used often depends upon the intended use and the specifications for the radio equipment. For example, dielectric and coaxial cavity resonator filters are often used for filtering electromagnetic energy in certain frequency bands, such as those used for cellular and PCS communications. Typically, such filter structures are implemented using a number of coupled dielectric or coaxial resonator structures. Coaxial dielectric resonators in such filters are coupled via capacitors, strip transmission lines, transformers, or by apertures in walls separating the resonator structures. The number of resonator structures used for any particular application also depends upon the system specifications. Increasing the number of intercoupled resonator structures improves performance in some application environments.
A conventional bandpass filter, for example, consists of several coaxial-type cavity resonators forming a multi-pole filter. A relatively large number of poles are used for adequate attenuation in the stop band at a given frequency distance from the pass band. As the number of poles increases, however, the insertion loss in the pass band also increases due to the loss of the resonators and cavities. While it is desirable to minimize the insertion loss, a lower insertion loss limits the number of poles. The stop band attenuation is also limited as a result. Thus, achieving low insertion loss in the pass band with higher attentuation in the stop band close to the pass band becomes a very challenging issue in some applications, for instance, cellular-phone communication.